Hermann Harde

Photoacoustic spectrometer based on a DFB-diode laser

We have developed a photoacoustic spectrometer based on a distributed feedback diode laser. The laser can be tuned continuously over 700 GHz enabling both the precise determinations of absorption line parameters such as the pressure broadening coefficient and pressure shift as well as sensitive concentration measurements.

Advanced Two-Layer ClimateModel for the Assessment of Global Warming by CO2

We present an advanced two-layer climate model, especially appropriate to calculate the influence of an increasing CO2-concentration and a varying solar activity on global warming. The model describes the atmosphere and the ground as two layers acting simultaneously as absorbers and Planck radiators, and it includes additional heat transfer between these layers due to convection and evaporation. The model considers all relevant feedback processes caused by changes of water vapour, lapse-rate, surface albedo or convection and evaporation. In particular, the influence of clouds with a thermally or solar induced feedback is investigated in some detail. The short- and long-wave absorptivities of the most important greenhouse gases water vapour, carbon dioxide, methane and ozone are derived from line-by-line calculations based on the HITRAN08-databasis and are integrated in the model. Simulations including an increased solar activity over the last century give a CO2 initiated warming of 0.2˚C and a solar influence of 0.54˚C over this period, corresponding to a CO2 climate sensitivity of 0.6˚C (doubling of CO2) and a solar sensitivity of 0.5˚C (0.1 % increase of the solar constant).

Photoacoustic spectrometer based on a Planckian radiator with fast time response

Abstract We have developed a photoacoustic spectrometer based on a fast responding Planckian radiator. This promising radiation source represents a matrix of 35 tiny resistors ( 0.500 mm ×0.357 mm) that emit their peak power in the mid-infrared wavelength region and that can be directly modulated via their current. It enables a simple, compact and robust detector for monitoring of trace gases like carbon dioxide (CO2).

Optoacoustic 13C-breath test analyzer

The composition and concentration of exhaled volatile gases reflects the physical ability of a patient. Therefore, a breath analysis allows to recognize an infectious disease in an organ or even to identify a tumor. One of the most prominent breath tests is the 13C-urea-breath test, applied to ascertain the presence of the bacterium helicobacter pylori in the stomach wall as an indication of a gastric ulcer. In this contribution we present a new optical analyzer that employs a compact and simple set-up based on photoacoustic spectroscopy. It consists of two identical photoacoustic cells containing two breath samples, one taken before and one after capturing an isotope-marked substrate, where the most common isotope 12C is replaced to a large extent by 13C. The analyzer measures simultaneously the relative CO2 isotopologue concentrations in both samples by exciting the molecules on specially selected absorption lines with a semiconductor laser operating at a wavelength of 2.744 μm. For a reliable diagnosis changes of the 13CO2 concentration of 1% in the exhaled breath have to be detected at a concentration level of this isotope in the breath of about 500 ppm.

Photoacoustic NO detection for asthma diagnostics

Exhaled nitric oxide was of high interest in breath analyses in the past few years. After its first detection in human breath in 1991, numerous publications uncovered the role of NO and its relation to different diseases. A strong relationship between an asthmatic eosinophilic airway inflammation and an increased NO level is medically confirmed. In this study a new photoacoustic detection system for nitric oxide based on a pulsed quantum cascade laser is introduced. The lasers single mode emission provides adequate selectivity to differentiate NO from other molecules in the sample. The demonstrated detection sensitivity allows in principle an application of the new system as diagnostic tool for asthma.

New Optical Analyzer for 13C-Breath Test

Medical breath tests are well established diagnostic tools, predominantly for gastroenterological inspections, but also for many other examinations. Since the composition and concentration of exhaled volatile gases reflect the physical condition of a patient, a breath analysis allows one to recognize an infectious disease in an organ or even to identify a tumor. One of the most prominent breath tests is the 13C-urea-breath test, applied to ascertain the presence of the bacterium helicobacter pylori in the stomach wall as an indication of a gastric ulcer. In this contribution we present a new optical analyzer that is based on photoacoustic spectroscopy and uses a DFB diode laser at 2.744 μm. The concentration ratio of the CO2 isotopologues is determined by measuring the absorption on a 13CO2 line in comparison to a 12CO2 line. In the specially selected spectral range the lines have similar strengths, although the concentrations differ by a factor of 90. Therefore, the signals are well comparable. Due to an excellent signal-noise-ratio isotope variations of less than 1% can be resolved as required for the breath test.

Photoacoustic sensor for VOCs: first step towards a lung cancer breath test

Development of new optical sensor technologies has a major impact on the progression of diagnostic methods. Specifically, the optical analysis of breath is an extraordinarily promising technique. Spectroscopic sensors for the non-invasive 13C-breath tests (the Urea Breath Test for detection of Helicobacter pylori is most prominent) are meanwhile well established. However, recent research and development go beyond gastroenterological applications. Sensitive and selective detection of certain volatile organic compounds (VOCs) in a patients breath, could enable the diagnosis of diseases that are very difficult to diagnose with contemporary techniques. For instance, an appropriate VOC biomarker for early-stage bronchial carcinoma (lung cancer) is n-butane (C4H10). We present a new optical detection scheme for VOCs that employs an especially compact and simple set-up based on photoacoustic spectroscopy (PAS). This method makes use of the transformation of absorbed modulated radiation into a sound wave. Employing a wavelength-modulated distributed feedback (DFB) diode laser and taking advantage of acoustical resonances of the sample cell, we performed very sensitive and selective measurements on butane. A detection limit for butane in air in the ppb range was achieved. In subsequent research the sensitivity will be successively improved to match the requirements of the medical application. Upon optimization, our photoacoustic sensor has the potential to enable future breath tests for early-stage lung cancer diagnostics.

Photoacoustic Sensor for Medical Diagnostics

The development of new optical sensor technologies has a major impact on the progress of diagnostic methods. Of the permanently increasing number of non-invasive breath tests, the 13C-Urea Breath Test (UBT) for the detection of Helicobacter pylori is the most prominent. However, many recent developments, like the detection of cancer by breath test, go beyond gastroenterological applications. We present a new detection scheme for breath analysis that employs an especially compact and simple set-up. Photoacoustic Spectroscopy (PAS) represents an offset-free technique that allows for short absorption paths and small sample cells. Using a single-frequency diode laser and taking advantage of acoustical resonances of the sample cell, we performed extremely sensitive and selective measurements. The smart data processing method contributes to the extraordinary sensitivity and selectivity as well. Also, the reasonable acquisition cost and low operational cost make this detection scheme attractive for many biomedical applications. The experimental set-up and data processing method, together with exemplary isotope-selective measurements on carbon dioxide, are presented.

Photoacoustic HF-Sensor

The development of new radiation sources has a major impact on the progress of optical trace gas detection. Particularly distributed feedback (DFB) diode lasers have proven to be very useful devices for spectroscopic sensors due to their spectral purity, direct modulation via the injection current, their small size and low acquisition cost. In this contribution we present a new sensor for hydrofluoric acid (HF), based on a room temperature DFB diode laser at 2.47 μm in combination with a photoacoustic detection scheme. The sensor enables detection of HF concentrations of less than 10 ppb with an excellent selectivity.

Photoacoustic CO2 detection at 2.7 μm

We present a new detection scheme for carbon dioxide(CO2) based on a custom-made room temperature distributed feedback (DFB) diode laser at 2.7 μm, currently representing the laser with the highest emission wavelength of its kind. The detectors especially compact and simple set-up is based on photoacoustic spectroscopy (PAS). This method makes use of the transformation of absorbed modulated radiation into a sound wave. The sensor enables a very high detection sensitivity for CO2 in the ppb range. Furthermore, the carefully selected spectral region as well as the narrow bandwidth and wide tunability of the single-mode laser ensure an excellent selectivity. Even measurements of different CO2 isotopes can be easily performed. This could enable future applications of this spectroscopic sensor in medical diagnostics (e.g. 13C-breath tests).